The present invention relates to a laser scanning unit which has an automatic power control function for stabilizing the intensity of an emitted laser beam.
Laser beam printers are well known and widely used as printers employing an electrophotographic imaging method. The laser printer employs a laser scanning unit for emitting a modulated laser beam which scans a surface to be scanned. Specifically, the laser scanning unit has a laser diode which is controlled to emit modulated laser beam. The laser beam is modulated on a dot basis (pixel basis) and the modulated laser beam scans a photoconductive surface in a predetermined direction. While the photoconductive surface is scanned by the modulated laser beam, the photoconductive surface is moved in a direction different from a scanning direction (e.g., in a direction perpendicular to the scanning direction). As a result, a certain area is exposed to the modulated laser beam and a two-dimensional latent image is formed on the photoconductive surface.
Recently a highly qualified image is demanded, and the printer is required to form not only a black and white image but a gradation image.
In order to vary a density of the image for froming the gradation image, energy to be applied for each pixel to the photoconductive surface is to be changed in accordance with the gradation level. Generally, there are two methods for producing the gradation image with the laser beam printer. One of the methods, which is generally used, is a method of varying the duration of time during which the laser beam is emitted (the laser diode is turned ON) without changing the intensity of the beam emitted by the laser diode. Usually, a pulse signal is applied to the laser diode to control the laser diode to emit the laser beam. The width of a pulse carried by the pulse signal corresponds to the duration of time during which the laser diode is turned ON. In this method, the pulse width is changed in accordance with a gradation level.
The other method is to vary the intensity of the emitted laser beam according to the density of the each pixel of the image without changing the width of the pulse. Recently, laser printers which are capable of performing a printing operation at a high speed are demanded. For the high speed printer, it is preferable to make the pulse width as short as possible. Therefore, in the latter method, the pulse width is fixed and the intensity of the laser beam emitted by the laser diode is varied in accordance with the image to be printed.
FIG. 5 shows an example of an Intensity-Current characteristic (referred to as an I-C characteristic) of a laser diode LD. The graph shows the intensity of the laser beam output by the laser diode, and an electrical current available through the laser diode. In this example, it is assumed that when the intensity of the laser beam is smaller, the density of the produced image is lower, and that when the intensity of the laser beam is greater, the density of the produced image is higher.
In the graph shown in FIG. 5, the intensity of the laser beam corresponding to a white image is indicated as Pw, and the intensity of the laser beam corresponding to a black image is indicated as Pb. The range of the intensity of the laser beam defined by the intensities Pw and Pb is divided into a predetermined number of gradations (e.g., 256), and according to the image data, the laser diode is controlled to emit a laser beam having one of the above intensities for each pixel. Specifically, in order to control the laser diode to emit the laser beam having the intensity corresponding to the density of the image to be produced, the quantity of the electrical current available in the laser diode is controlled in accordance with the image data.
However, the I-C characteristic varies depending on the ambient temperature. That is, if the temperature changes, the intensity of the laser beam with respect to the electrical current available through the laser diode varies, and therefore, an entire image may not have a stable (constant) density. The I-C characteristic changes such that the inclinations of the characteristic lines do not change but the line shifts in a right- or left-hand direction in FIG. 5, an example being shown by broken lines.
Further, the characteristic has a threshold current value Ith. If the electrical current available through the laser diode is greater than the threshold current value Ith, the laser diode performs stably. That is, the performance of the laser diode is guaranteed, and the intensity of the laser diode is substantially proportional to the electrical current flowing through the laser diode. However, if the electrical current available through the laser diode is less than the threshold value Ith, the performance of the laser diode is unstable, and the intensity of the laser beam may not correspond to the flowing current.
Furthermore, the threshold value Ith varies depending on the temperature, as mentioned above (indicated Ith' as an example in FIG. 5). In order to obtain a gradation image, the intensity of the laser beam should be strictly controlled, and therefore the above-described variation of the I-C characteristic should be taken into account when the laser diode is driven. In other words, the intensity of the laser beam when a white image is drawn should be maintained as Pw, and in order to obtain the gradation image, the difference between the intensity Pb for a black image and the intensity Pw for the white image should be maintained to have a predetermined difference.
Still further, even if the intensity of the laser beam is adjusted so as to correspond to the desired gradation, if the sensitivity of the photoconductive surface changes, the image having the desired gradation cannot be obtained. The sensitivity of the photoconductive drum may change due to a change of the ambient temperature, deterioration of the photoconductive material covering the peripheral surface of the drum, exchange of the photoconductive drum, and the like. Further, even if the characteristic of the photoconductive is unchanged, when the photoconductive drum is exposed to light of various color components, the intensity of the beam may be changed for respective color components.